This invention relates to prepurifiers employed in gas separation systems and, more particularly, to apparatus for improving gas distribution flow within such prepurifiers.
Prepurifiers are used in cryogenic plants to reduce water, carbon dioxide and hydrocarbon inclusions in inlet air to ppm levels, prior to the air entering a cold box. The prepurifier keeps the water and carbon dioxide from freezing and plugging heat exchangers and aid in the safe operation of air separation plants. Prepurifiers adsorb the gases in alumina and 13X molecular sieve beds. The adsorbents are contained in, for example, horizontal vessels which are operated in alternating adsorption and regeneration cycles.
A horizontal prepurifier 10 is illustrated in FIG. 1a and positions bed 12 in the middle of vessel 14. An inlet air distributor 16 and an outlet air accumulator 18 are positioned in vessel 14 on opposite sides of adsorbent bed 12. Horizontal beds can be very large, i.e., up to 60xe2x80x2 long by 16xe2x80x2 diameter. Flow mal-distribution in prepurifiers can cause early breakthrough, loss of efficiency, sieve movement and local bed fluidization. The air flow patterns of horizontal bed 10 are shown in FIG. 1b. 
There are two main types of mal-distribution in horizontal beds, namely axial, and radial. Axial mal-distribution is due to high flow mass exit of flow from the end of current distributors. FIG. 1c illustrates a prior art configuration used in the prepurifier of FIGS. 1a and 1b, where gas distributor 16 and gas accumulator 18 are perforated and have closed distal ends.
FIG. 1d is a chart that illustrates gas flow in distributor 16 of FIG. 1a and shows that more of the gas flow exits at 80% of the axial distance from the centerline of vessel 14. Such unbalanced flow can impact bed 12 at the end of the distributor and cause a sieve movement.
The prior art has utilized 50% perforated distributor shown in FIG. 1c along the length of the vessel for inlet and outlet flows of the prepurifier. It has been determined that up to 38% axial mal-distribution can be expected with this type of distributor for feed flows in a 16 ft dia., 60 ft length vessel.
Radial mal-distribution is a function of geometry of the bed. In radial mal-distribution, flow next to the vessel shell at the entrance or exit to bed is at a higher velocity than the flow in the center of the bed due to vessel curvature. A radial mal-distribution can be avoided by shallow beds or removing vessel curvature with an internal wall.
The prior art includes a number of teachings regarding solutions to flow distribution in vessels. U.S. Pat. No. 5,779,773 to Cam et al. illustrates a retaining grille in a vessel with a convex bottom. The grille has a curved shape and is located over the inlet pipe. The direction of convexity of the vault and the bottom of the vessel are opposite to each other obtaining a good diffusion of the gas. The grille is used to support an adsorbent bed in the vessel. The application of a grille for vertical and horizontal beds is discussed.
U.S. Pat. No. 5,716,427 to Andreani et al. describes a perforated region flow distributor for use on adsorbent beds which are generally of a radial flow design. The perforated region divides the annular channels into two sections, using variable size holes or a variable hole distribution to provide uniform flow to the adsorbent bed. In a second case, a perforated cylindrical bed support region diffuses the inlet jet and results in uniform flow to the outer channel.
European Patent Application EP 0748253 to Zardi et al. illustrates a vertical mixing assembly bed for gaseous flows at different temperatures for heterogeneous exothermic synthesis processes. The device has mixing assemblies to cool the partially reacted gas mixtures flowing through the bed and comprises a large vertical collector which runs from the top to the bottom of the bed at the center of vessel. The bed has several compartments which are fed by different assemblies.
U.S. Pat. No. 5,544,423 to Westelaken et al. describes a gas distributor with spatially separate gas deflectors. Each deflector positioned to successively cut the flow diagonally across the gas flow. The deflectors are configured to direct gas flow exiting the distributor in an essentially perpendicular direction to the direction of gas flow entering the distributor. A dryer for particulate material includes particulate material, particulate material inlets and outlets, a drying gas inlet and an exhaust gas outlet.
U.S. Pat. No. 4,938,422 to Koves describes an inlet distributor for vertical vessels. A two directional flow generator prevents bed surface disturbances at high inlet velocities and high particle loadings. The distributor uses a series of partitions to peel off portions of the downward gas flow and to redirect them radially outward. Each outwardly directed gas flow component passes through a series of perforations to effect any necessary circumferential redistribution before entering the space above the particle bed. By subdividing the gas flow into a number of radially directed flow portions and circumferentially redistributing these flow portions, cross-currents and eddy currents on the catalyst bed surface are minimized.
U.S. Pat. No. 4,065,391 to Farabaugh illustrates a distributor which supplies either a liquid or a gas to a bed of granular media. The vessel is divided by inclined interior walls into primary and secondary horizontal conduits that extend parallel to each other. A liquid or a gas is supplied to the primary conduits, passes through the metering orifices into the secondary conduits, and through dispersion orifices into the filter bed. The system uses an air backwashing prior to conventional liquid washing. The air bubbles up through the filter and provides a thorough agitation of the media, dislodging accumulated dirt and/or gelatinous floc which then can be removed by liquid backwashing.
U.S. Pat. No. 5,298,226 to Jeffert et al. provides uniform fluid flow in pressure swing adsorbent vertical vessels. The vessel has at least two perforated region fluid distributors positioned transversely with respect to a longitudinal axis of the vessel.
U.S. Pat. No. 5,538,544 to Jeffert et al. describes a non-uniform graded ball adsorbent bed support that is employed in the head section of an adsorption vertical vessel to achieve a uniform flow of gas in bed. The open area of the inlet gas distributor is modified to channel gas toward the edge portion of the vessel head to enhance the uniform flow of gas to the adsorbent bed.
There remains a need for an improved air distributor that is of simple construction and provides a relatively uniform air distribution throughout a prepurifier vessel.
A distributor apparatus arrangement distributes a gas within a vessel that includes an adsorbent bed, the gas distribution occurring with a minimum of gas mal-distribution. The distributor apparatus includes an inlet gas coupler that feeds gas to a conduit oriented adjacent a surface of the bed. A slit opening is positioned in a wall of the conduit and extends along a length of the conduit. The slit has an opening extent near the inlet fluid coupler that is larger than an opening extent of the slit near the distal end of the conduit. The slit opening is oriented towards a wall of the vessel and away from the bed so as to enable gas flow that exits therefrom to impact upon the wall and be deflected thereby to the bed. In a preferred arrangement, the slit is defined by perforations that exhibit a 50% aperture ratio.